1. Field of Invention
The present invention relates to a packet transmitting method. More particularly, the present invention relates to a packet transmitting method of wireless network.
2. Description of Related Art
Recently, wireless networks, such as WLAN and WMAN, have been developed one after the other, and networks have been setup one by one to provide services. Wherein packet voice/video is regarded as one of the most important application service on wireless network system. However, the provision of real-time communication services on wireless network system experiences many challenges. For example, wireless hand-held devices usually use batteries as a power supply so that the power consumption problem has to be resolved. In addition, video and audio data of real-time communication system is usually timely generated as a small packet, which may greatly affect bandwidth utilization of wireless network system.
Most wireless network system are designed for transmitting data packets, but not optimized for the transmission of real-time packets. The characteristics of real-time communication packet are not fully utilized in the design of wireless network system, for example, usually real-time communication can tolerate packet loss to certain extent, and using additional bandwidth to achieve reliable transmission is not the most important consideration in audio packet transmission. Thus, increasing network bandwidth and reducing power consumption of handheld devices is able to facilitate the development of the real-time communication services of wireless network environment.
The operation flow of standard 802.11 is as shown in FIG. 1, when a mobile station (MS) transmits a packet, if the wireless media is used by other MS, the MS waits until the wireless media is free and then waits for a DCF interframe space (DIFS), then the MS begins to count down the contention window (CW). When the CW is counted down to 0, the MS starts to transmit control packets such as RTS (request transmission send), CTS (confirm transmission send) etc, or data packet and fragmentation packets (such as Frg#1 and Frg#2). After receiving the correct packet, a receiving terminal transmits an ACK packet to the transmitting terminal after a short interframe space (SIFS).
FIG. 2 is a system configuration diagram of a wireless network using real-time communication service. Referring to FIG. 2, in the wireless network, one or a plurality of wireless network devices 202 and 204 are connected to a cable network through wireless network AP (access point) 210 to access the Internet 230. For example, an uplink real-time frame is transmitted to a router 220 in local network through the wireless network AP 210, and then a router 220 transmits the packet to the other correspondent node 240 capable of real-time communication through the Internet 230. As to a downlink real-time frame, the packet is first transmitted to the router 220 through the Internet 230, and then the router 220 transmits the packet to the wireless network device 202 or 204 through the wireless network AP 210.
Presently, the simplest method for transmitting real-time packet through wireless network is turning on a network card all the time so that the network card can always receive packet. However, in such design, the network card still consumes power even when it does not receive packet. One solution to ameliorate power consumption is to turn on the wireless network to a sleeping mode, and the wireless network device is waken up when there is a packet to be transmitted. Such method is referred to as a PS-Poll transmission mechanism. FIG. 3 is a diagram illustrating the packet timing and the corresponding power consumption while transmitting real-time packet in wireless network with the PS-Poll transmission mechanism. Referring to FIG. 3, the wireless network device (STA) begins transmitting uplink and downlink real-time frames after performing the related connecting and registering actions. After the uplink real-time packet (UL Voice Packet) is transmitted, the wireless network device continues to wait for a response packet (ACK) of the wireless network AP because the wireless network device has to determine whether the packet needs to be re-transmitted. In addition, the downlink real-time packet (DL Voice Packet) stored in the buffer of the wireless network AP needs to be taken over through the PS-Poll because the wireless network device is in a power saving mode. Similarly, to allow the wireless network AP to determine whether to re-transmit the packet, the wireless network device has to transmit the response packet to the wireless network AP, so that the wireless network AP can confirm that the packet has been received successfully. Accordingly, to reliably transmit real-time packet, two short interframe spaces (SIFS) and two response packets are necessitated which may limit the improvement in power consumption and network-utilized bandwidth.
If unscheduled-automatic power saving delivery (U-APSD) mechanism defined in 802.11e is used for transmitting real-time packets, the power consumption of the PS-Poll transmission mechanism can be further reduced. FIG. 4 is a diagram illustrating packet timing and its corresponding power consumption while transmitting real-time packet in wireless network with the U-APSD mechanism. Compared with FIG. 3, FIG. 4 is similar to FIG. 3 except there is one PS-Poll packet fewer in FIG. 4, so the similar part isn't described again.
To achieve better power saving performance, many relative media control technologies have been further developed besides the foregoing power saving delivery. However, some of the power saving mechanisms may change the transmission mechanism of 802.11 and thus incompatible with the standard. Some of the power saving mechanisms are limited by the detection or the result of the estimation so may have different power saving performances. Moreover, some of the power saving mechanisms may consume more power in other portions while saving power in a particular portion and some other power saving mechanisms can cause other problems, such as reduced throughput or disconnection, to achieve power saving performance.
To avoid the aforementioned disadvantages, the inventor provided a technology for transmitting real-time communication packets in multicast manner. And this technology is proved to be efficient in reducing power consumption. However, the transmission of multicast packets does not perform packet acknowledgement in the standard specification; thus, there still needs improvement in ensuring communication quality.
In other words, all the works have been done so far to save the power have their own disadvantages, especially some power saving mechanisms may cause inconvenience in application due to their incompatibility with the standard. Thus, the existing technology cannot resolve the problems in real-time communication effectively.